Conventional semi-additive processes (SAPs) commonly laminate a photosensitive solder resist (SR) material on to metal layers that are on one (or both) sides of a dielectric material. An electrical connection with the metal layer(s) is typically formed through the SR material by exposing the SR material with ultraviolet radiation to cross-link selected (i.e., patterned) areas on the SR material. The unexposed areas on the SR material are then usually removed to form solder resist openings (SRO).
Reducing the size and pitch of the SROs is typically quite difficult because of the complex interplay between material properties, exposure, and development tools. One of these difficulties includes UV light scattering on rough pad (e.g., copper) surfaces which limits the accuracy of the SR footprint thereby making it difficult to minimize the size of the SRO. In addition, the numerical aperture of the UV projection tool also limits the resolution of the SR footprint thereby making it difficult to minimize the size of the SRO.
Another common difficulty relates to limited etching solution flow within the SROs. This limited etching solution flow commonly prevents full development of high aspect ratio SROs.
Another common approach to fabricating SROs relates to utilizing UV laser drilling. UV laser drilling can potentially create 20 um SRO. However, UV laser drilling is typically very slow and negatively impacts the throughput of electronic assemblies that are fabricated using UV drilling due to the slow UV laser drilling. Any reduction in throughput may negatively impact the cost associated with fabricating electronic assemblies.
Many electronic assemblies include SROs of more than one size. These types of electronic assemblies are typically fabricated utilizing two different process steps. The first process step commonly uses standard UV exposure to form large SROs. The second process step usually uses standard UV laser drilling to form small SROs. The need for different process steps to fabricate electronic assemblies that include different sized SROs decreases manufacturing throughput due to the slow UV laser drilling and the need for multiple process steps.